In recent years, in order to increase the image forming speed in a color image forming apparatus of an electrophotography system, color image forming apparatuses of a tandem system, each of which comprises developers and photosensitive drums in a number as large as the number of color materials, and transfers images of different colors on an image conveyor belt or print medium, are increasing. In such a color image forming apparatus of a tandem system, there are a plurality of factors that cause registration errors (color discrepancy), and various strategies against respective factors have been proposed.
For example, one such factor is non-uniformity or a mounting position shift of a lens used in an exposure scan device used to expose each photosensitive drum. Another factor is an assembly position shift of the exposure scan device to the color image forming apparatus main body. With these factors, a laser scan line that exposes each photosensitive drum (image carrier) deforms with respect to the photosensitive drum, and if such deformation varies for respective colors, registration errors occur. As one of the strategies against such registration errors, a method described in patent reference 1 is known. With this method, the curvature of a laser scan line that exposes each photosensitive drum with respect to the photosensitive drum is measured in an assembly process of a polarization scan device, and the mounting position of the lens with respect to a polarization scan device is adjusted based on the measurement result.
Patent reference 2 discloses another method. In this method, in an assembly process of the exposure scan device in the color image forming apparatus main body, the deformation of a laser scan line that exposes each photosensitive drum with respect to the photosensitive drum is measured, and the assembly position of the exposure scan device to the color image forming apparatus main body is adjusted based on the measurement result.
Furthermore, patent reference 3 describes a method which measures the deformation of a laser scan line that exposes each photosensitive drum with respect to the photosensitive drum, corrects bitmap image data to cancel the measured deformation, and forms an image based on the corrected image data. This method can cope with registration errors with lower cost than the methods described in patent references 1 and 2, since it can electrically correct image data by processing it and can obviate the need for mechanical adjustments.    Patent reference 1: Japanese Patent Laid-Open No. 2002-116394    Patent reference 2: Japanese Patent Laid-Open No. 2003-241131    Patent reference 3: Japanese Patent Laid-Open No. 2004-170755